Back contact design for solar cell, and method of fabricating same

ABSTRACT

A method includes depositing spacers at a plurality of locations directly on a back contact layer over a solar cell substrate. An absorber layer is formed over the back contact layer and the spacers. The absorber layer is partially in contact with the spacers and partially in direct contact with the back contact layer. The solar cell substrate is heated to form voids between the absorber layer and the back contact layer at the locations of the spacers.

FIELD

This disclosure relates to solar cells generally, and in particular tothin film photovoltaic cells and methods of fabricating the same.

BACKGROUND

Photovoltaic cells or solar cells are photovoltaic components for directgeneration of electrical current from sunlight. Due to the growingdemand for clean sources of energy, the manufacture of solar cells hasexpanded dramatically in recent years and continues to expand. Solarcells include a substrate, a back contact layer on the substrate, anabsorber layer on the back contact layer, a buffer layer on the absorberlayer, and a front contact layer above the buffer layer. The layers canbe applied onto the substrate during a deposition process using, forexample, sputtering and/or co-evaporation.

Semi-conductive materials are used in at least a portion of the absorberlayer of some solar cells. For example, chalcopyrite basedsemi-conductive materials, such as copper indium gallium selenide (CIGS)(also known as thin film solar cell materials), are used to complete theformation of the absorber layer after the deposition process.

In semiconductor materials, the term “recombination,” refers to aphenomenon in which an electron recombines with a hole giving off excessenergy to a second electron instead of emitting the energy as a photon.The second electron (and successive electrons) then give up theadditional energy in a series of collisions, relaxing back to the edgeof the band. Thus, the effect is a result of interactions betweenmultiple particles, including multiple electrons and a hole. The neteffect is that many electron-hole pairs, which could otherwise generateuseful power, recombine, and the charge carriers are eliminated.

Because recombination is based on the ability of the charge carriers toexchange energy, the probability of recombination increases with ahigher concentration of charge carriers.

In highly concentrated sunlight, recombination significantly reducessolar cell efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a solar cell according to anembodiment of this disclosure.

FIG. 2 is a cross sectional view of the solar cell of FIG. 1, takenacross section line 2-2 of FIG. 1, according to some embodiments.

FIG. 3 is a cross sectional view of the solar cell of FIG. 1, takenacross section line 2-2 of FIG. 1, according to other embodiments.

FIG. 4 is a scanning electron microscope photograph showing the layersof the solar cell of FIG. 1.

FIG. 5 is a flow chart of a method of fabricating the solar cell of FIG.1.

FIG. 6A is a flow chart of one embodiment of the spacer deposition stepin FIG. 5.

FIG. 6B is a flow chart of another embodiment of the spacer depositionstep in FIG. 5.

FIG. 7 is a diagram showing improved power generation by the solar cellof FIG. 1.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the drawings, like referencenumerals indicate like items.

In the description, relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description anddo not require that the apparatus be constructed or operated in aparticular orientation.

This disclosure describes a variety of photovoltaic cells, in whichspacers or voids are provided directly on the top surface of the backcontact layer. The spacers or voids reduce the contact area between theabsorber layer and the back contact layer, providing a solar cell withreduced recombination and higher solar efficiency. Methods offabricating the solar cells are also described.

FIG. 1 is a cross sectional view of a solar cell 100 according to someembodiments. The solar cell 100 includes a solar cell substrate 102, aback contact layer 104, a an absorber layer 107, a plurality of voids106 in the absorber layer, a buffer layer 108 and a front contact layer110.

Substrate 102 can include any suitable substrate material, such asglass. In some embodiments, substrate 102 includes a glass substrate,such as soda lime glass, or a flexible metal foil or polymer (e.g., apolyimide, polyethylene terephthalate (PET), polyethylene naphthalene(PEN)). Other embodiments include still other substrate materials.

Back contact layer 104 includes any suitable back contact material, suchas metal. In some embodiments, back contact layer 104 can includemolybdenum (Mo), platinum (Pt), gold (Au), silver (Ag), nickel (Ni), orcopper (Cu). Other embodiments include still other back contactmaterials. In some embodiments, the back contact layer 104 is from about50 nm to about 2 μm thick.

In some embodiments, absorber layer 107 includes any suitable absorbermaterial, such as a p-type semiconductor. In some embodiments, theabsorber layer 107 can include a chalcopyrite-based material comprising,for example, Cu(In,Ga)Se₂ (CIGS), cadmium telluride (CdTe), CulnSe₂(CIS), CuGaSe₂ (CGS), Cu(In,Ga)Se₂ (CIGS), Cu(In,Ga)(Se,S)₂ (CIGSS),CdTe or amorphous silicon. Other embodiments include still otherabsorber materials. In some embodiments, the absorber layer 107 is fromabout 0.3 μm to about 3 μm thick.

The absorber layer 107 contains a plurality of voids 106 directly on thetop surface of the back contact layer. As used herein, the term “void inthe absorber layer” refers to a volume 106 within the absorber layer107, in which the absorber layer material is absent. In someembodiments, the voids 106 contain a vacuum. In some embodiments, thevoids 106 contain a spacer material, described below. In someembodiments, the voids 106 contain one or more precursors for formingthe spacer material. In some embodiments, the voids 106 contain one ormore residual products from decomposition of the spacer material. Insome embodiments, the voids include a combination of one or more of avacuum, a spacer material, a spacer material precursor and/or a residualproduct of spacer material decomposition. Thus, the term “voids in theabsorber” encompasses voids 106 in the absorber layer 107, with orwithout a material in the voids. The voids 106 provide insulating spaceor insulating material between portions of the absorber layer 107 andthe underlying portions of the back contact layer 104. The voids 106provide discontinuities in the conductive interface between the absorberlayer 107 and the back contact layer 104.

In some embodiments, the voids 106 contain spacers formed of aninsulating spacer material, such as an oxide. In some embodiments, thespacer material comprises silicon dioxide. In some embodiments, thespacer material comprises a metal oxide, such as TiO, TaO, Al₂O₃, ZrO₂,MoO₂, or BaTiO₃. In some embodiments, the spacer material comprises ahigh resistance compound semiconductor, such as HfO₂. In someembodiments, the spacer material comprises particles having sizes fromabout 50 nm to about 1,000 nm. In some embodiments, the spacer materialcomprises particles having sizes from about 100 nm to about 500 nm. Inother embodiments, the spacer material comprises nanoparticles havingsizes from about 1 nm to about 100 nm.

In some embodiments, from 10% to 80% of the bottom surface of theabsorber layer 107 contacts the voids 106 or spacers within the voids.In some embodiments, from about 90% to about 20% of a bottom surface ofthe absorber layer 107 is in direct contact with the back contact layer104, and a remainder of the absorber layer 107 confronts either voids106 or insulating spacers within the voids.

In some embodiment, the spacers cover from about 70% to about 80% of theback contact layer 104 with spacer material, and about 30% to about 20%of the absorber layer 107 in direct contact with the underlying backcontact layer 104. The inventors have determined that a solar cell 100having about 80% of the absorber layer 107 abutting the voids 106 andabout 20% of the absorber layer directly contacting the back contactlayer has a solar module efficiency that is about 105% to 106% of thesolar module efficiency of an otherwise similar solar cell having novoids 106 in the absorber layer 107. The solar cell 100 having about 80%of the absorber layer 107 abutting the voids 106 and about 20% of theabsorber layer directly contacting the back contact layer 104 has noincrease in the risk of absorber delamination.

Buffer layer 108 includes any suitable buffer material, such as n-typesemiconductors. In some embodiments, buffer layer 108 can includecadmium sulphide (CdS), zinc sulphide (ZnS), zinc selenide (ZnSe),indium(III) sulfide (In₂S₃), indium selenide (In₂Se₃), orZn_(1-x)Mg_(x)O, (e.g., ZnO). Other embodiments include still otherbuffer materials. In some embodiments, the buffer layer 108 is fromabout 1 nm to about 500 nm thick.

In some embodiments, front contact layer 110 includes an annealedtransparent conductive oxide (TCO) layer. In some embodiments, the TCOlayer 110 is highly doped. For example, the charge carrier density ofthe TCO layer 110 can be from about 1×10¹⁷ cm⁻³ to about 1×10¹⁸ cm⁻³.The TCO material for the annealed TCO layer can include any suitablefront contact material, such as metal oxides and metal oxide precursors.In some embodiments, the TCO material can include zinc oxide (ZnO),cadmium oxide (CdO), indium oxide (In₂O₃), tin dioxide (SnO₂), tantalumpentoxide (Ta₂O₅), gallium indium oxide (GaInO₃), (CdSb₂O₃), or indiumoxide (ITO). The TCO material can also be doped with a suitable dopant.In some embodiments, ZnO can be doped with any of aluminum (Al), gallium(Ga), boron (B), indium (In), yttrium (Y), scandium (Sc), fluorine (F),vanadium (V), silicon (Si), germanium (Ge), titanium (Ti), zirconium(Zr), hafnium (Hf), magnesium (Mg), arsenic (As), or hydrogen (H). Inother embodiments, SnO2 can be doped with antimony (Sb), F, As, niobium(Nb), or tantalum (Ta). In other embodiments, In₂O₃ can be doped withtin (Sn), Mo, Ta, tungsten (W), Zr, F, Ge, Nb, Hf, or Mg. In otherembodiments, CdO can be doped with In or Sn. In other embodiments,GaInO3 can be doped with Sn or Ge. In other embodiments, CdSb₂O₃ can bedoped with Y. In other embodiments, ITO can be doped with Sn. Otherembodiments include still other TCO materials and corresponding dopants.In some embodiments, the front contact layer 110 is from about 5 nm toabout 3 μm thick outside of the P2 scribe line, from about 0.5 nm toabout 3 μm on side walls of the P2 scribe line, and from about 5 nm toabout 3 μm on the bottom of the P2 scribe line (directly on the backcontact layer 104).

Solar cell 100 also includes interconnect structures that include threescribe lines, referred to as P1, P2, and P3. The P1 scribe line extendsthrough the back contact layer 104 and is filled with the absorber layermaterial. The P2 scribe line extends through the buffer layer 108 andthe absorber layer 107 and is filled with the front contact layermaterial. The P3 scribe line extends through the front contact layer110, buffer layer 108 and absorber layer 107.

FIG. 2 is a cross-sectional view of the solar cell 100 taken alongsection line 2-2 of FIG. 1, looking down on the back contact layer 104and voids 106. The absorber layer 107 comprises an absorber layermaterial over the back contact layer 104, where the absorber layermaterial is partially in direct contact with the back contact layer 104,and the absorber layer material has a plurality of voids 106 therein.The voids 106 are located directly on the back contact layer 104.

In the embodiment of FIG. 2, the voids 106 are distributed uniformly orsubstantially uniformly over the top surface of the back contact layer104 (except in the scribe line regions P1, P2 and P3). A uniformdistribution of voids 106 as shown in FIG. 2 can be formed by depositinga uniform spacer material film and performing a photolithography step,as discussed below in the description of FIG. 6B. A uniform distributionof voids 106 provides good process control. In some embodiments, thespacer density is in a range from 1×10⁸ to 4×10¹⁰ spacers/cm².

FIG. 3 shows another embodiment of a solar cell 200, in which the voidsare randomly distributed over the top surface of the back contact layer104 (except in the scribe line regions P1, P2 and P3). A randomdistribution of voids 106 as shown in FIG. 3 can be formed by spraying aspacer material directly on the back contact layer 104, as discussedbelow in the description of FIG. 6A. The spacer density for a sprayedspacer material can be the same as for a patterned spacer material, forexample, in a range from 1×10⁸ to 4×10¹⁰ spacers/cm². Randomlydistributed spacer material can be deposited inexpensively by a sprayingprocess.

FIG. 4 is a scanning electron microscope (SEM) image showing a detail ofthe cross section of the solar cell 100 of FIG. 1. The back contactlayer 104, voids 106, absorber layer 107, buffer layer 108 and frontcontact layer 110 are shown. In FIG. 4, the voids 106 are substantiallyfree of solid materials. In some embodiments, the spacer materialdeposited on the back contact layer 104 is an insulating material havingan evaporation temperature below the processing temperatures used whendepositing and/or annealing one or more of the remaining layers(absorber layer 107, buffer layer 108, front contact layer 110). Forexample, if the vaporization temperature of the spacer material is at orbelow 400° C. (or at or below 600° C.), then the spacer material can besubstantially removed from the interface between the absorber 107 andthe back contact layer 104, leaving vacant voids 106, as shown in FIG.4. The remaining direct contact area between the absorber 107 and theback contact layer 104 occupies more than 5% of the total surface areaof the back contact layer 104. In some embodiments, the remaining directcontact area between the absorber 107 and the back contact layer 104occupies more than 10% of the total surface area of the back contactlayer 104. In some embodiments, the remaining direct contact areabetween the absorber 107 and the back contact layer 104 occupies about20% of the total surface area of the back contact layer 104.

FIG. 5 is a flow chart of a method of fabricating the solar cell 100 ofFIG. 1.

At step 502, a back contact layer 104 is formed over a solar cellsubstrate. In some the back contact layer 104 can deposited bysputtering a metal such as molybdenum over the solar cell substrate 102.At the conclusion of back contact layer deposition, the P1 scribe lineis formed (e.g., scribed or etched) through the back contact layer 104.

At step 504, spacers 106 are deposited at a plurality of locationsdirectly on the back contact layer 104 over the solar cell substrate102. Details of some embodiments of the spacer deposition step areprovided below with reference to FIGS. 6A and 6B.

At step 506, the absorber layer 107 is formed over the back contact 104and spacers 106. The bottom of absorber layer 107 partially contacts thespacers 106 and partially contacts the back contact layer 104. In someembodiments, the absorber comprises CIGS. In some embodiments, aplurality of CIGS precursors are sputtered onto the spacers 106 andexposed portion of the back contact layer 104. In some embodiments, theCIGS precursors include Cu/In, CuGa/In and/or CuInGa, applied bysputtering. The absorber layer material fills the P1 scribe line.Following the sputtering of these precursors, selenization is performed.

At step 508, the buffer layer 108 is formed over the absorber layer 107.For example, in some embodiments, a layer of CdS, ZnS or InS is formedby chemical bath deposition (CBD). In other embodiments, the bufferlayer 108 is deposited by sputtering or atomic layer deposition (ALD).Following the deposition of the buffer layer 108, the P2 scribe line isformed (e.g., scribed or etched) through the absorber layer 107 andbuffer layer 108.

At step 510, the front contact layer 110 is formed over the bufferlayer. In some embodiments, the front contact layer 110 is i-ZnO or AZOapplied by sputtering. In other embodiments, the front contact layer 110is BZO applied by metal organic chemical vapor deposition (MOCVD). Thefront contact layer material conformally coats the side and bottom wallsof the P2 scribe line. Following deposition of the front contact layer110, the P3 scribe line is formed (e.g., scribed or etched) through thefront contact layer, buffer layer 108, and absorber layer 107.

At step 512, in some embodiments the solar cell substrate 102 is heatedto form voids 106 between the absorber layer 107 and the back contactlayer 104 at the locations of the spacers. In some embodiments, theheating step includes heating the substrate to a temperature from about400° C. to about 600° C. In some embodiments, the heating step 512 isaccomplished by an annealing step. In some embodiments, the heating step512 is included in a step of annealing the front contact (window) layer110. Thus, the heating step 512 can be achieved without adding anadditional annealing step to the solar cell fabrication process. In someembodiments, following the heating step, the voids 106 contain a vacuum,a spacer material, a spacer material precursor, a spacer materialdecomposition residual product, or a combination of the above. Forexample, in some embodiments, after the annealing step, the voids 106only contain a vacuum or a spacer material decomposition residualproduct at a low (partial vacuum) pressure.

FIG. 6A is a flow chart showing an embodiment of step 504 of FIG. 5. InFIG. 6A, step 504 includes step 600 of spring spacer material directlyon the back contact layer 104. In some embodiments, the spacer materialparticles 106 have sizes from about 100 nm to about 500 nm. In someembodiments, the step of depositing spacers 106 includes sprayingnanoparticles on the back contact layer. In some embodiments, the stepof depositing spacers 106 includes spraying silicon dioxide particleshaving sizes from about 100 nm to about 500 nm on about 70% to about 80%of the back contact layer 104.

The step of spraying spacer material can be integrated with the solarcell production line. For example, in some embodiments, the spraying isperformed by one or more nozzles within the same process chamber inwhich the back contact layer 104 is deposited. A plurality of nozzlescan be used to improve the uniformity of the spacer particles.

FIG. 6B is a flow chart of an alternative embodiment of step 504 of FIG.5. In the embodiment of FIG. 6B, step 504 includes steps 610-616.

At step 610, a spacer material film is deposited on the back contactlayer 104. In some embodiments, the spacer material film is depositeduniformly. Uniform material deposition processes can include one or moreof physical vapor deposition (PVD), chemical vapor deposition (CVD),sputtering, ALD, epitaxial formation, or the like. In some embodiments,a uniform silicon dioxide film is deposited.

At step 612, a photoresist is deposited on the spacer material film.

At step 614, the photoresist is patterned. The photoresist isselectively exposed through a photomask, and the exposed photoresist isdeveloped and baked. A positive or negative photoresist can be used,with the appropriate photomask. The portions of the photoresist in thespacer locations is rendered (or remains) insoluble, and the portions ofthe photoresist outside of the spacer locations is rendered (or remains)soluble. The soluble photoresist is dissolved and removed, leaving ahard mask comprising resist portions in the spacer locations.

At step 616, the spacer material film is etched back outside of thespacer locations, leaving a uniform field of spacers 106, as shown inFIG. 2.

FIG. 7 is a diagram showing the open circuit voltage (Voc) improvementdue to the inclusion of the voids and/or spacers 106 directly on theback contact layer, underlying a portion of the absorber layer. Curve702 corresponds to a solar cell without the voids/spacers describedabove. Curve 704 corresponds to a solar cell with the voids/spacersdescribed above. Table 1 summarizes relevant performance data comparingthe solar cells without and with the voids/spacers.

TABLE 1 CdS/CIGS Jsc Seff (cm/s) (mA/cm2) Voc (V) F.F. (%) Eff. (%) With100 34.11 0.6907 82.98 19.55 voids/spacers No 100,000 33.97 0.6657 81.9518.54 voids/spacers

The first data column of Table 1 (CdS/CIGS Seff (cm/s)) provides surfacerecombination velocity (Seff) for the solar cell of FIG. 1 (withvoids/spacers) and a control solar cell (no voids/spacers).Recombination velocity on a semiconductor surface is defined as theratio of the normal component of the electron (or hole) current densityat the surface to the excess electron (or hole) charge density at thesurface. The portion of the graph corresponding to the current densityrange (along the Y-axis) from about 32 to 37 (corresponding to the levelof illumination from the sun) is of particular interest. The value ofSeff with spacers is about 100 cm/s, and the value of Seff with nospacers is about 100,000 cm/s. Thus, the inclusion of spacers reducessurface recombination velocity by three orders of magnitude. Table 1further shows that the short circuit current Jsc is improved from 33.97mA/cm² without voids/spacers to 34.11 mA/cm² with voids/spacers. Opencircuit voltage Voc increases from 0.6657 V without voids/spacers to0.6907 V with voids/spacers. The fill factor increases from 0.8195without voids/spacers to 0.8298 with voids/spacers. The fill factor isdefined as the ratio of the actual maximum obtainable power to theproduct of the open circuit voltage and short circuit current. The fillfactor describes the curvature of the current/voltage curves 702, 704,and higher fill factor is an indicator of improved performance.

Because recombination removes electron-hole pairs from the absorber,this 1,000× reduction in recombination velocity can increase overallsolar cell efficiency (Eff.) from 18.54% (without voids/spacers) to19.55%. This represents about a 5.4% improvement in solar cellefficiency.

Examples are described above for providing voids/spacers directly incontact with the back contact layer 104 of a p-n junction type solarcell. The void/spacer method described herein can be used with otherthin film solar cells, including but not limited to a-Si thin film solarcells, CIGS and CdTe solar cells with p-n junctions, p-intrinsic-n(p-i-n) structure, metal-insulator-semiconductor (MIS) structure,multijunction (e.g., p-n-p-n or p-n-p-n-p-n structure with two or threeabsorbers stacked in series), or the like.

In some embodiments, a method comprises: depositing spacers at aplurality of locations directly on a back contact layer over a solarcell substrate; forming an absorber layer over the back contact layerand the spacers, the absorber layer partially in contact with thespacers and partially in direct contact with the back contact layer; andheating the solar cell substrate to form voids between the absorberlayer and the back contact layer at the locations of the spacers.

In some embodiments, the step of depositing spacers includes sprayingspacer material particles on the back contact layer.

In some embodiments, the spacer material comprises a metal oxide.

In some embodiments, the spacer material comprises silicon dioxide.

In some embodiments, the spacer material particles have sizes from about100 nm to about 500 nm.

In some embodiments, step of depositing spacers includes sprayingnanoparticles on the back contact layer.

In some embodiments, the step of depositing spacers includes: depositinga silicon dioxide film on the back contact layer; and usingphotolithography to remove portions of the silicon dioxide film outsideof the locations of the spacers.

In some embodiments, the step of depositing spacers includes coveringfrom about 70% to about 80% of the back contact layer with spacermaterial.

In some embodiments, the heating step includes heating the substrate toa temperature from about 400° C. to about 600° C.

In some embodiments, the step of depositing spacers includes sprayingsilicon dioxide particles having sizes from about 100 nm to about 500 nmon about 70% to about 80% of the back contact layer; and the heatingstep includes heating the substrate to a temperature from about 400° C.to about 600° C.

In some embodiments, a method comprises: spraying spacer material at aplurality of locations directly on a back contact layer over a solarcell substrate; and forming an absorber layer over the back contactlayer and the spacer material, so that from about 10% to about 80% ofthe absorber layer is in direct contact with the spacer material andfrom about 90% to about 20% of the absorber layer is in direct contactwith the back contact layer.

Some embodiments further comprise heating the solar cell substrate toform voids between the absorber layer and the back contact layer at thelocations of the spacers.

In some embodiments, the spacer material comprises silicon dioxide.

In some embodiments, the spacer material comprises particles havingsizes from about 100 nm to about 500 nm.

In some embodiments, a solar cell, comprises: a solar cell substrate; aback contact layer over the solar cell substrate; an absorber layercomprising an absorber layer material over the back contact layer, theabsorber layer material partially in direct contact with the backcontact layer, the absorber layer material having a plurality of voidstherein, the voids located directly on the back contact layer; a bufferlayer over the absorber layer; and a front contact layer over the bufferlayer.

Some embodiments further comprise spacers directly on the back contactlayer within the plurality of voids.

In some embodiments, the spacers comprise an insulating material.

In some embodiments, the spacers comprise particles having sizes fromabout 100 nm to about 500 nm.

In some embodiments, from about 20% to about 90% of a bottom surface ofthe absorber layer is in direct contact with the back contact layer, anda remainder of the absorber layer confronts at least one of the groupconsisting of the voids or insulating spacers in the voids.

In some embodiments, the voids are randomly distributed on the backcontact layer.

Although the subject matter has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodiments,which may be made by those skilled in the art.

What is claimed is:
 1. A method comprising: depositing spacers at aplurality of locations directly on a back contact layer over a solarcell substrate; forming an absorber layer over the back contact layerand the spacers, the absorber layer partially in contact with thespacers and partially in direct contact with the back contact layer; andheating the solar cell substrate to form voids between the absorberlayer and the back contact layer at the locations of the spacers.
 2. Themethod of claim 1, wherein the step of depositing spacers includesspraying spacer material particles on the back contact layer.
 3. Themethod of claim 2, wherein the spacer material comprises a metal oxide.4. The method of claim 2, wherein the spacer material comprises silicondioxide or high resistance compound semiconductor.
 5. The method ofclaim 2, wherein the spacer material particles have sizes from about 100nm to about 500 nm.
 6. The method of claim 1, wherein step of depositingspacers includes spraying nanoparticles on the back contact layer. 7.The method of claim 1, wherein the step of depositing spacers includes:depositing a silicon dioxide film or isolation film on the back contactlayer; and using photolithography to remove portions of the silicondioxide film or isolation film outside of the locations of the spacers.8. The method of claim 1, wherein the step of depositing spacersincludes covering from about 70% to about 80% of the back contact layerwith spacer material.
 9. The method of claim 1, wherein the heating stepincludes heating the substrate to a temperature from about 400° C. toabout 600° C.
 10. The method of claim 1, wherein: the step of depositingspacers includes spraying silicon dioxide particles having sizes fromabout 100 nm to about 500 nm on about 70% to about 80% of the backcontact layer; and the heating step includes heating the substrate to atemperature from about 400° C. to about 600° C.
 11. A method comprising:spraying spacer material at a plurality of locations directly on a backcontact layer over a solar cell substrate; and forming an absorber layerover the back contact layer and the spacer material, so that from about10% to about 80% of the absorber layer is in direct contact with thespacer material and from about 90% to about 20% of the absorber layer isin direct contact with the back contact layer.
 12. The method of claim11, further comprising heating the solar cell substrate to form voidsbetween the absorber layer and the back contact layer at the locationsof the spacers.
 13. The method of claim 11, wherein the spacer materialcomprises silicon dioxide.
 14. The method of claim 11, wherein thespacer material comprises particles having sizes from about 100 nm toabout 500 nm.
 15. A method comprising: spraying spacer material at aplurality of locations directly on a back contact layer over a solarcell substrate; forming an absorber layer over the back contact layerand the spacer material, so that from about 10% to about 80% of theabsorber layer is in direct contact with the spacer material; andheating the solar cell substrate to form voids between the absorberlayer and the back contact layer at the locations of the spacermaterial.
 16. The method of claim 15, wherein the heating step includesheating the substrate to a temperature from about 400° C. to about 600°C.
 17. The method of claim 15, wherein the spacer material comprisessilicon dioxide.
 18. The method of claim 15, wherein the step ofdepositing spacers includes spraying nanoparticles on the back contactlayer.
 19. The method of claim 15, wherein the spraying is performed byone or more nozzles within a same process chamber in which the backcontact layer is deposited over the solar cell substrate.